The present invention relates to a device for controlling the transmission of a vehicle, and, more particularly, to a device for controlling the shift timing of the transmission of a vehicle in accordance with the operating conditions of the engine and the vehicle, from the standpoint of maximizing fuel economy and power output performance.
In a vehicle equipped with a manual transmission and a mechanical clutch, the fuel economy is generally expressed by the following formula: EQU Ef=Sv.times.wf.times.et.times.1000/(fb.times.Nr) (1)
where:
Ef is the fuel economy, in km/liter; PA0 Sv is the vehicle speed, in km/hour; PA0 wf is the specific gravity of the fuel, in g/cm.sup.3 ; PA0 et is the mechanical efficiency of the gear train; PA0 fb is the brake specific fuel consumption, in g/PS.hour; and PA0 Nr is the required power, in PS.
In order to maximize the fuel economy, under the condition that the vehicle speed Sv and the required power Nr are constant, it is necessary to minimize the value fb/(wf.times.et). In this case, since the specific gravity of the fuel wf and the mechanical efficiency of the geartrain et can be taken as being substantially constant, regardless of the operating conditions of the engine, it is necessary to minimize the specific fuel consumption fb, in order to maximize the fuel economy.
Substituting the fuel consumption F (in liters/hour) for the brake specific fuel consumption fb, the above is simplified as follows: EQU Ef=Sv/F (2)
Under the condition that the required power Nr is constant, the fuel consumption F is proportional to the brake specific fuel consumption fb, and therefore the condition for minimizing the fuel consumption F can be substituted for the condition for minimizing the brake specific fuel consumption fb. Now, because the brake specific fuel consumption fb varies in relation to engine speed and engine torque, better fuel consumption will be achieved if engine speed is controlled while maintaining the required power. The optimum condition for this control is given by a performance line in a graph taking engine torque and the engine speed as variables. Hereinafter this performance line is referred to as the maximum fuel economy operation line. In order to change engine speed under the condition that the vehicle speed Sv and the required power Nr are maintained at constant values, it is necessary to change transmission ratio, namely to shift the transmission. Therefore, with regard to the transmission there exists the optimum shift point which enables the engine to operate at the required performance, and if shifting of the transmission is performed at this point, the engine comes to be operated along the abovementioned maximum fuel economy operation line, thereby accomplishing high fuel economy.
In the case of a vehicle equipped with an automatic transmission including a fluid torque converter, the equation for the fuel economy of the vehicle is, in consideration of the torque converter efficiency ec, modified as follows: EQU Ef=Sv.times.wf.times.ec.times.et.times.1000/(fb.times.Nr) (3)
The torque converter efficiency ec varies, as is well known, in accordance with the transmission ratio e (e=turbine shaft speed/pump shaft speed). Contour lines of various required powers Nr (equi-Nr lines), which are drawn on the graph which takes engine torque and engine speed as variables, are different from those of a manual transmission, especially in the low engine speed range, and therefore the maximum fuel economy operation line of the vehicle differs from that of a vehicle equipped with a manual transmission.
The maximum fuel economy operation line of a vehicle equipped with an automatic transmission including a fluid torque converter has not been heretofore clarified. However, we have found this as a result of experimental researches. Namely, it has been found that, in the high engine speed range, the maximum fuel economy operation line of a vehicle equipped with an automatic transmission is approximately the same as that of a vehicle equipped with a manual transmission, while in the low engine speed range, the line of the vehicle having an automatic transmission extends along the speed ratio curve which maximizes the converter efficiency ec.